linux-kernel - Re: [patch 0/4] [RFC] Another proportional weight IO controller

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Message-ID: <20081118144139.GE15268@gandalf.sssup.it>
Date:	Tue, 18 Nov 2008 15:41:39 +0100
From:	Fabio Checconi <fchecconi@...il.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Nauman Rafique <nauman@...gle.com>, Li Zefan <lizf@...fujitsu.com>,
	Divyesh Shah <dpshah@...gle.com>,
	Ryo Tsuruta <ryov@...inux.co.jp>, linux-kernel@...r.kernel.org,
	containers@...ts.linux-foundation.org,
	virtualization@...ts.linux-foundation.org, jens.axboe@...cle.com,
	taka@...inux.co.jp, righi.andrea@...il.com, s-uchida@...jp.nec.com,
	fernando@....ntt.co.jp, balbir@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, menage@...gle.com, ngupta@...gle.com,
	riel@...hat.com, jmoyer@...hat.com, peterz@...radead.org,
	paolo.valente@...more.it
Subject: Re: [patch 0/4] [RFC] Another proportional weight IO controller

> From: Vivek Goyal <vgoyal@...hat.com>
> Date: Tue, Nov 18, 2008 09:07:51AM -0500
>
> On Tue, Nov 18, 2008 at 01:05:08PM +0100, Fabio Checconi wrote:
...
> > I have to think a little bit on how it would be possible to support
> > an option for time-only budgets, coexisting with the current behavior,
> > but I think it can be done.
> > 
> 
> IIUC, bfq and cfq are different in following manner.
> 
> a. BFQ employs WF2Q+ for fairness and CFQ employes weighted round robin.
> b. BFQ uses the budget (sector count) as notion of service and CFQ uses
>    time slices.
> c. BFQ supports hierarchical fair queuing and CFQ does not.  
> 
> We are looking forward for implementation of point C. Fabio seems to
> thinking of supporting time slice as a service (B). It seems like
> convergence of CFQ and BFQ except the point A (WF2Q+ vs weighted round
> robin). 
> 
> It looks like WF2Q+ provides tighter service bound and bfq guys mention
> that they have been able to ensure throughput while ensuring tighter 
> bounds. If that's the case, does that mean BFQ is a replacement for CFQ
> down the line?
>   

BFQ started from CFQ, extending it in the way you correctly describe,
so it is indeed very similar.  There are also some minor changes to
locking, cic handling, hw_tag detection and to the CIC_SEEKY heuristic.

The two schedulers share similar goals, and in my opinion BFQ can be
considered, in the long term, a CFQ replacement; *but* before talking
about replacing CFQ we have to consider that:

  - it *needs* review and testing; we've done our best, but for sure
    it's not enough; review and testing are never enough;
  - the service domain fairness, which was one of our objectives, requires
    some extra complexity; the mechanisms we used and the design choices
    we've made may not fit all the needs, or may not be as generic as the
    simpler CFQ's ones;
  - CFQ has years of history behind and has been tuned for a wider
    variety of environments than the ones we've been able to test.

If time-based fairness is considered more robust and the loss of
service-domain fairness is not a problem, then the two schedulers can
be made even more similar.
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